Process for purifying aqueous crude ethanol solution

ABSTRACT

Disclosed is a purification process for removing lipophilic impurities  coined in an aqueous crude ethanol solution, in particular, for efficiently removing impurities consisting of C 3  -C 4  alcohols. The process involves (a) a first extraction step wherein the aqueous crude ethanol solution is extracted with an extractant for removing the lipophilic impurities, except for the C 3  -C 4  alcohols, contained in the aqueous crude ethanol solution by subjecting the aqueous crude ethanol solution to extraction with the extractant in a pressurized state containing carbon dioxide in a liquidized state or carbon dioxide gas in a supercritical state; (b) a concentration-distillation step wherein a raffinate obtained in the first extraction step is fed to a distilling column to thereby obtain a highly concentrated aqueous ethanol solution from a top of the distilling column and withdraw fraction containing C 3  -C 4  alcohols from the distilling column as a side stream; (c) a second extraction step wherein the side stream is brought into contact with the extract in a pressurized state, obtained in the first extraction step to thereby extract the C 3  -C 4  alcohols contained in the side stream into the extract; and (d) a water washing step wherein the extract in a pressurized state, obtained in the second extraction step, is brought into countercurrent contact with water in a pressurized state under such a condition that a ratio of the weight of water to the weight of the extract is set to 0.3 or less to thereby recover the ethanol contained in the extract into an aqueous phase.

TECHNICAL FIELD

The present invention relates to a process for purifying an aqueouscrude ethanol solution.

BACKGROUND ART

Ethanol may be prepared by fermenting sugars such as molasses or byhydrating ethylene (hereinafter referred to sometimes as fermentationmethod and synthesis method, respectively).

Ethanol preparable by the fermentation method or by the synthesis methodis an aqueous crude ethanol solution which is contaminated with variouskinds of impurities. Major impurities contained in the aqueous crudeethanol solution prepared by the fermentation method include a varietyof compounds, such as methanol, acetaldehyde, n-propanol, n-butanol,ethyl acetate, 3-methylbutanol, and so on. On the other hand, majorimpurities contained in the aqueous crude ethanol solution prepared bythe synthesis method also include a variety of compounds, such asacetaldehyde, diethylether, acetone, sec.-butanol, n-butanol,crotonaldehyde, and so on.

As described hereinabove, the various kinds of impurities are containedin the aqueous crude ethanol solution, yet they are so minute in amountthat the removal of such impurities is very difficult. In particular,alcohols having three or four carbon atoms (hereinafter referred to asC₃ -C₄ alcohols) are so similar in their physicochemical properties toethanol that it is extremely difficult to remove them, as compared withthe other impurities.

The distillation has generally been employed to purify an aqueous crudeethanol solution containing the various kinds of the sugars as describedhereinabove. This distillation method requires a large number ofdistilling columns and it consumes a large quantity of steam fordistillation. Further, at least one of the plurality of the distillingcolumns is operated by the extractive distillation method in such amanner that a crude ethanol once concentrated by distillation is addedagain to a large volume of water and the resulting mixture is fed to theextractive distilling column.

In addition, the C₃ -C₄ alcohols among the impurities can be removed bywithdrawing a side stream from the side of the distilling column forconcentrating the ethanol; however, a large amount of the ethanol isalso withdrawn from the distilling column so that the withdrawn sidestream is further distilled in vacuo to thereby recover the ethanol.

Hence, the conventional process for the purification of ethanol using alarge number of distilling columns is very poor in efficiency of energy.

In order to improve the problem with the poor efficiency in energy asseen in the distillation method as described hereinabove, a process isproposed in Japanese Patent Laid-open Publication (kokai) Nos. 60-41,627and 2-49,741, which involves removing lipophilic impurities contained inan aqueous crude ethanol solution by extraction with carbon dioxide in aliquidized state or with carbon dioxide gases in a supercritical state(hereinafter referred sometimes to as carbon dioxide extraction method).

By the term "lipophilic impurities" referred to herein is meantoxygen-containing compounds having two or more carbon atoms, except forethanol. The aqueous crude ethanol solution preparable by thefermentation method contains methanol as a hydrophilic impurity, inaddition to the lipophilic impurities.

The carbon dioxide extraction method can separate the lipophilicimpurities as an extract and provide a mixed aqueous solution of ahighly purified ethanol and methanol as a raffinate. The rate ofextraction (removal) of the lipophilic impurities in this case increasesin proportion to an increase in a ratio of an extractant (that is, aratio of the weight of the extractant to the weight of the aqueous crudeethanol solution), and the predetermined lipophilic impurities can beextracted as the ratio of the extractant reaches to a somewhat largeextent. The minimal ratio of the extractant may vary with theconcentration of ethanol in the aqueous crude ethanol solution, thecomposition or the concentration of the impurities, the acceptable rangeof the impurities in purified ethanol, or the like. Further, the higherthe ratio of the extractant, the larger the amount of the extractant tobe employed; hence, a high pressure extraction column and a compressorshould be made so large in scale and size that costs of equipment becomeexpensive and costs of power required for the compression and thecirculation of carbon dioxide gases become high.

It can be noted herein that the C₃ -C₄ alcohols are the lipophilicimpurities contained in the aqueous crude ethanol solution, that are themost difficult to extract upon the extraction with carbon dioxide in theliquidized state or carbon dioxide gases in the supercritical state. Inthe conventional processes, a considerably large amount of the C₃ -C₄alcohols are left unremoved in the raffinate although almost all amountsof the other lipophilic impurities can be extracted at a certain ratioof the extractant; hence, the ratio of the extractant should beincreased to a higher extent in order to extract all the amounts of thelipophilic impurities.

The present invention has the object to provide a process for thepurification of an aqueous crude ethanol solution, so adapted as toefficiently separate and remove C₃ -C₄ alcohols from the aqueous crudeethanol solution without causing any increase in the ratio of theextractant.

DISCLOSURE OF INVENTION

More specifically, the present invention is to provide a process for thepurification of the aqueous crude ethanol solution, which comprises: (i)a first extraction step wherein the aqueous crude ethanol solution isextracted with an extractant for removing the lipophilic impurities,except for the C₃ -C₄ alcohols, contained in the aqueous crude ethanolsolution by subjecting the aqueous crude ethanol solution to extractionwith the extractant in a pressurized state, which comprises carbondioxide in a liquidized state or carbon dioxide gas in a supercriticalstate; (ii) a concentration-distillation step wherein a raffinateobtained in the first extraction step is fed to a distilling column tothereby obtain a highly concentrated aqueous ethanol solution from a topof the distilling column and withdraw a fraction containing C₃ -C₄alcohols from the distilling column as a side stream; (iii) a secondextraction step wherein the side stream is brought into contact with theextract in a pressurized state, obtained in the first extraction step,to thereby extract the C₃ -C₄ alcohols contained in the side stream intothe extract; and (iv) a water washing step wherein the extract in apressurized state, obtained in the second extraction step, is broughtinto countercurrent contact with water in a pressurized state under sucha condition that a ratio of the weight of the water to the weight of theextract is set to 0.3 or less to thereby recover the ethanol containedin the extract into an aqueous phase.

The present invention can advantageously be applied to either aqueouscrude ethanol solution, whether it is an aqueous ethanol solutionprepared by the fermentation method or a roughly distilled aqueousethanol solution prepared by distilling the aqueous ethanol solutionprepared by the fermentation method. The aqueous ethanol solutionobtained by the fermentation method may generally contain ethanol at therate of from 10% to 40% by weight and, when the roughly distilledaqueous crude ethanol solution is employed, it is employed as it is orit is employed in the form of an aqueous solution in which theconcentration of the ethanol is diluted to from 10% to 50% by weight.

The extractant to be employed for the present invention may includecarbon dioxide in the liquidized state or carbon dioxide gases in thesupercritical state. For the process according to the present invention,it is preferred to use the carbon dioxide sustained at pressure in therange of from 40 to 150 kg/cm² and at temperature in the range of from20° C. to 50° C.

Now, a description will be made of the ratio of the extractant in thefirst extraction step with reference to the results shown in Table 1 forworking examples as will be described hereinafter. The rate at which thelipophilic impurities contained in the aqueous crude ethanol solutionare extracted is increased in accordance with an increase in the ratioof the extractant. For example, the lipophilic impurities, except forpropanol and butanol, can be extracted and removed at the rate of 98.5%or higher when the ratio of the extractant is set to 2.0 and at the rateof 99.9% or higher when the ratio of the extractant is set to 4.0. Onthe other hand, propanol and butanol can be removed at the rate of 79%or lower when the ratio of the extractant is set to 2.0 and, when theratio of the extractant is to set 4.0, butanol can be removed at therate of 99% or higher yet propanol can be removed at the rate ofapproximately 73%. In order to allow propanol to be removed at the rateof 98.5% or higher, however, it is needed to raise the ratio of theextractant to 8 or higher. Hence, it can be understood that the ratio ofthe extractant at which only the lipophilic impurities except forpropanol and butanol can be extracted at the rate of 98.5% is sufficientto be a quarter of the ratio of the extractant of 8, at which all thelipophilic impurities containing propanol and butanol can be removed atthe rate of 98.5%.

It can be noted that the first extraction step of the process accordingto the present invention is operated with the main purpose to remove thelipophilic impurities, except for the C₃ -C₄ alcohols, contained in theaqueous crude ethanol solution, so that the conditions are adopted inthe first extraction step so as to comply with a desired rate of removalof the lipophilic impurities except for the C₃ -C₄ alcohols. Generally,the condition is such that the ratio of the extractant is in the rangeof from 2 to 5, preferably from 3 to 4.5. Accurately, however, the ratioof the extractant should be selected by experimental data because therate of extraction varies to a subtle extent in accordance with theconcentration of ethanol and impurities contained in the aqueous crudeethanol solution, the temperature or pressure of extraction, or thelike.

The raffinate obtained in the first extraction step is subjected todistillation after gases were removed. The removal of the gases iscarried out by removing the carbon dioxide gases, used as theextractant, by vaporization of the carbon dioxide in the liquidizedstate. The modes of distillation may vary with the concentration ofethanol of the raffinate, and it is usually carried out at ambientpressure or at reduced pressure of approximately 500 mmHg through a traytype distilling column having a theoretical number of nearly thirtystages. In this mode of distillation, a fraction rich in a C₃ alcohol orin the C₃ -C₄ alcohols is withdrawn from a middle stage of thedistilling column as a side stream, and a highly concentrated mixture ofaqueous solution of highly purified ethanol and methanol is extractedfrom the top of the distilling column. The material to be withdrawn asthe side stream is a mixture of a liquid phase with a gaseous phase.Further, by appropriately selecting the reflux ratio and the amount ofthe mixture to be withdrawn, the C₃ -C₄ alcohols can be removed to asufficient extent and the concentration of the C₃ -C₄ alcohols containedin the mixed aqueous solution of ethanol and methanol, obtained from thecolumn top, can be sustained within an acceptable range ofconcentration.

In the process according to the present invention, in particular, when acrude ethanol having the concentration of ethanol in the range of from80% to 90% is employed as a raw material, the raffinate containingethanol at the concentration of 30% to 40% can be obtained from a bottomof the first extracting column, so that the process according to thepresent invention can present the advantage that the load imposed uponthe distilling column is lowered and the distilling column can be madesmaller in size. In this case, it can further offer the advantages thatenergy can be saved by allowing a heat source having a relatively lowquality, such as low pressure steam, and a highly efficient heat pump tobe employed, because a side reboiler for the distilling column can beheated with a heat source having a relatively low temperature.

In the process according to the present invention, the side stream isfed at elevated pressure to the second extraction step, and it isbrought into countercurrent contact with the extract in the pressurizedstate, obtained in and fed from the first extraction step. In the secondextraction step, the C₃ -C₄ alcohols contained in the side stream areseparated as the extract and a portion of the ethanol is recovered as araffinate.

The ratio of the extractant in the second extraction step, i.e. a ratioof the weight of the extract from the first extraction step to theweight of the side stream from the distillation step, can appropriatelybe determined in accordance with the ratio of the extractant in thefirst extraction step and the amount of the withdrawn side stream. Inother words, a large majority of the carbon dioxide fed in the firstextraction step is extracted as the extract in the first extractionstep, while the side stream in the distillation step usually amounts forapproximately 20% to 30% of the aqueous crude ethanol solution, so thatthere is employed the ratio of the extractant of from approximately 6.0to 20.0, which is three to five times the ratio of the extractant forthe first extraction step. Hence, the rate of removal of the C₃ -C₄alcohols may become as high as 90% or higher, as will become apparentfrom Table 3 for the working examples as will be described hereinafter.To the contrary, however, the quantity of ethanol accompanying theextract increases, thereby resulting in an increase in the quantity ofthe ethanol which will be lost by following the extract in the finalwater washing step. Hence, for the process according to the presentinvention, a portion or an entire portion of the aqueous phase of thewater washing step is mixed with the side stream and the mixture isrecirculated to the second extraction step or only a portion of theextract from the first extraction step is fed to the second extractionstep, while the rest of the extract is adjusted for its ratio of theextractant by feeding it to the water washing step or by any otherappropriate procedures. Finally, the extract withdrawn from the secondextraction step is fed to the water washing step in such a state that itis pressurized, and it is brought into countercurrent contact with waterto thereby recover ethanol. In this stage, a ratio of the weight of thewashing water to the weight of the extract may usually be set to 0.3 orlower, preferably to from 0.02 to 0.10.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram suitable for carrying out the according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, an aqueous crude ethanol solution containing ethanoland impurities is introduced into an upper portion of a first extractingcolumn 1 through a line 5, while an extractant consisting of carbondioxide gases in a pressurized state is introduced into a lower portionof the first extracting column 1 through a line 6.

In the first extracting column 1, the aqueous crude ethanol solution isallowed to come into countercurrent contact with the extractant.

The ratio of the extractant (the ratio of the weight of the extractantto the weight of the aqueous crude ethanol solution) within the firstextracting column 1 is so selected as for the rate of extraction of allthe lipophilic impurities, except for the C₃ -C₄ alcohols, to amount toa predetermined rate of removal or higher. The ratio of the extractantis usually set to 2 or higher, preferably from 3 to 4.5.

At the conditions as described hereinabove, the substantially entirequantity of the lipophilic impurities, except for the C₃ -C₄ alcohols,contained in the aqueous crude ethanol solution is allowed to migrateinto the extract phase to thereby provide the extract consisting of thelipophilic impurities other than the C₃ -C₄ alcohols, a portion of theC₃ -C₄ alcohols, water, ethanol, and a small quantity of methanol.

The resulting extract is then introduced in a pressurized state into thelower portion of a second extracting column 2 through a line 7. On theother hand, a raffinate phase consisting of water, ethanol, methanol andthe remaining unextracted C₃ -C₄ alcohols is remained at the bottom ofthe first extracting column 1. The raffinate phase is then withdrawnfrom a line 8 and fed to a concentration-distillation column 4 after thegases were removed.

Then, by appropriately selecting the reflux ratio of theconcentration-distillation column 4 and the quantity of the side streamto be withdrawn from the middle stage of the column, a highlyconcentrated ethanol is withdrawn through a line 16 from a top of theconcentration-distillation column 4, the highly concentrated ethanolbeing substantially free from the lipophilic impurities yet containing asmall quantity of methanol. At the same time, a fraction containing thesubstantially entire quantity of the C₃ -C₄ alcohols remaining in theraffinate phase fed from the line 8 is withdrawn from the middle stageof the concentration-distillation column 4 through a line 9 as a sidestream. Further, exhaust liquid is withdrawn through a line 17 from thebottom of the column.

The side stream is fed to the top of the second extracting column 2through the line 9, and it is allowed to come into countercurrentcontact with the extract fed from the first extracting column 1 throughthe line 7 into the bottom of the second extracting column 2. The ratioof the extractant (the ratio of the weight of the extract fed from thefirst extracting column 1 to the weight of the side stream) in thecountercurrent contact is basically determined by the quantity of theextractant fed into the first extracting column 1 and the quantity ofthe side stream in the concentration-distillation column 4, and it is inthe range of usually from 6 to 20, preferably from 6 to 12. In the rangeof the ratio of the extractant as described hereinabove, the C₃ -C₄alcohols within the side stream can be extracted into the extract at therate of 90% or higher. On the other hand, a considerably large quantityof ethanol contained in the side stream is recovered as a raffinate intowhich a small amount of the lipophilic impurities is in turn extractedfrom the extract existing in the line 7.

The extract obtained from the top of the second extracting column 2 isfed to a water washing column 3 while it is in a pressurized statethrough a line 10. A raffinate is withdrawn from the bottom of the waterwashing column 3 through a line 15, and preferably at least a portion ofthe raffinate is recirculated to the top of the first extracting column1 through a line 20.

In the water washing column 3, the extract fed from the line 10 into thebottom of the water washing column 3 in the pressurized state is allowedto come into countercurrent contact with washing water fed from a line18. The extract fed from the line 10 into the water washing column 3contains ethanol extracted in the first and second extracting columns,and the countercurrent contact allows the ethanol existing in theextract to be extracted into the washing water. The weight ratio ofwater (the ratio of the weight of water to the weight of the extract) inthe water washing column 3 is usually set to 0.3 or below, preferably inthe range of from 0.02 to 0.1. After washing with water, the extract iswithdrawn from the top of the water washing column 3 through a line 11.On the other hand, the washing water is withdrawn from the bottom of thewater washing column 3 through a line 12. In the water washing column 3,however, a small quantity of the C₃ -C₄ alcohols as well as ethanol areextracted into the washing water so that, preferably, at least a portionof the washing water is mixed with the side stream of the line 9 whilepassing through a line 21 and it is then fed to the second extractingcolumn 2.

Bypass lines 13 and 14 may be employed in order to appropriately adjustthe ratio of the extractant in the second extracting column 2 and theweight ratio of water in the water washing column 3. Further, water maybe introduced from a line 19 into the side stream in order to adjust theconcentration of ethanol in the second extracting column 2.

In addition, in the process according to the present invention, theconcentration of the aqueous ethanol solution to be fed to thedistilling column 4 may reach 30% to 40% by weight, so that a sidereboiler 20 may be provided and arranged to be heated with a low qualityheat source. As a result, such a low quality heat source and a highlyefficient heat pump 21 may be employed.

EXAMPLES

The present invention will be described in more detail by way ofexamples.

EXAMPLE 1

(a) First extraction step:

A sample solution similar to an aqueous crude ethanol solutionobtainable by fermentation method yet containing somewhat moreimpurities was extracted in the first extraction step in which a column,38 mm in inner diameter and 3 meters in height, having a 1/4 inchRaschig ring filled therein, was employed as a first extracting column.As the sample solution, there was employed a solution in which anaqueous ethanol solution having the concentration of 40% by weight ofethanol was mixed with each of impurities consisting of methanol,isoamyl alcohol, n-propanol, n-butanol, ethyl acetate, and acetaldehydeat the rate of 0.2% by weight.

The sample aqueous solution was fed from the top of the first extractingcolumn at the rate of 500 grams per hour, and an extractant consistingof carbon dioxide gases in a supercritical state was fed from the lowerportion of the first extracting column so as to provide a predeterminedratio of the weight of the extractant to the weight of the aqueoussolution (G/L). The pressure within the column was set to 100 kg percm², and the temperature within the column as a whole was maintained at40° C.

In order to analyze the ingredients of the extract, the extract waswithdrawn from the top of the first extracting column and an aqueousethanol solution was obtained by treating the extract at reducedpressure and vaporizing the extractant only. On the other hand, araffinate was withdrawn from the bottom of the first extracting column,and an aqueous ethanol solution was obtained by treating the raffinateat reduced pressure and vaporizing the extractant only.

Table 1 below shows rates (percentage by weight) of extraction of theingredient of the impurities contained in the extract as obtained in theextraction experiments as described hereinabove.

                                      TABLE 1                                     __________________________________________________________________________    G/L                                                                           (wt/wt)                                                                            1.0                                                                              2.0 3.0                                                                              4.0 5.0                                                                              6.0 7.0                                                                              8.0 9.0                                                                              10.0                                      __________________________________________________________________________    EtCHO                                                                              7.70                                                                             15.40                                                                             23.10                                                                            30.80                                                                             38.50                                                                            46.20                                                                             53.90                                                                            61.56                                                                             69.04                                                                            76.07                                     MeCHO                                                                              72.87                                                                            98.65                                                                             99.77                                                                            99.91                                                                             99.95                                                                            99.97                                                                             99.97                                                                            99.98                                                                             99.98                                                                            99.99                                     i-AmOH                                                                             85.04                                                                            99.44                                                                             99.88                                                                            99.95                                                                             99.97                                                                            99.98                                                                             99.98                                                                            99.98                                                                             99.99                                                                            99.99                                     n-BtOH                                                                             40.42                                                                            79.27                                                                             96.46                                                                            99.15                                                                             99.68                                                                            99.84                                                                             99.90                                                                            99.93                                                                             99.95                                                                            99.96                                     n-PrOH                                                                             18.40                                                                            36.80                                                                             55.19                                                                            73.05                                                                             86.98                                                                            94.32                                                                             97.39                                                                            98.67                                                                             99.24                                                                            99.52                                     Et--Ace                                                                            99.94                                                                            99.98                                                                             99.99                                                                            99.99                                                                             99.99                                                                            99.99                                                                             99.99                                                                            99.99                                                                             99.99                                                                            99.99                                     MeOH 3.90                                                                             7.80                                                                              11.70                                                                            15.60                                                                             19.50                                                                            23.40                                                                             27.30                                                                            31.20                                                                             35.10                                                                            39.00                                     __________________________________________________________________________

(b) Distillation & separation of raffinate from first extraction step:

By using a distilling column of a batch type, 3.8 mm in inner diameterand 3 meters in height, having a Dickson packing filled therein, theraffinate obtained at the G/L of approximately 3.0 in the firstextraction step was distilled to thereby recover ethanol. Theconcentration of ethanol in the raffinate in this case was approximately27% by weight, and the concentration of propanol was 0.1% by weight.

The distillation was carried out at the reflux ratio of approximately 3in the distilling column and the fractions withdrawn one after anotherfrom the top of the column, each amounting to the volume ofapproximately 300 cm², were analyzed for their ingredients. The analysisresults are shown in Table 2 below. In this distillation treatment,fractions Nos. 1 to 3 were recovered as a purified aqueous ethanolsolution, and fraction No. 4 was recovered as a side stream.

It can be noted herein that, as far as this distillation treatment isconcerned, an aqueous ethanol solution of an extremely high purity,containing extremely small (undetectable) quantities of propanol andbutanol, can be yielded, if only the fractions Nos. 1 and 2 arerecovered as a purified aqueous ethanol solution. In this case, thefractions Nos. 3 and 4 are to be recovered as a side stream.

                  TABLE 2                                                         ______________________________________                                                                               Volume of                              Fraction                               Fraction                               No.    Ethanol  Propanol Butanol                                                                              Methanol                                                                             (mm.sup.2)                             ______________________________________                                        1      90.4     --       --     3.8    300                                    2      92.1     --       --     2.0    300                                    3      89.6     0.005    --     0.5    300                                    4      61.0     0.9      0.042  0.2    300                                    5      3.0      0.001    --     --     300                                    6      1.0      --       --     --     300                                    ______________________________________                                    

(c) Second extraction step:

The fraction No. 4 obtained in the separation by distillation asdescribed in paragraph (b) above was employed as a side stream in thesecond extraction step in which a column, 3.8 mm in inner diameter and 3meters in height, having Raschig rings filled therein, was employed as asecond extracting column. In the second extracting column, the extractobtained in the first extraction step at the G/L ratio of 3.0 was fed atthe rate of 1,200 grams per hour into the bottom of the secondextracting column. The side stream was fed from the top of the secondextracting column so as to provide a predetermined ratio of theextractant (the ratio of the weight of the extract to the weight of theside stream). The pressure within the column was set to 100 kg per cm²and the temperature within the column as a whole was maintained at 40°C. The extract was withdrawn from the top of the second extractingcolumn, and the lipophilic impurities were recovered by treating theextract at reduced pressure and vaporizing the extractant only. On theother hand, the raffinate was withdrawn from the bottom of the column,and an aqueous ethanol solution was recovered by treating the raffinateat reduced pressure and vaporizing the extractant only. Table 3 belowshows rates of extraction of the ingredients of the impurities (% byweight of each of the ingredients) contained in the extract as obtainedin the extraction experiments as described hereinabove.

                  TABLE 3                                                         ______________________________________                                        G/L                                                                           (wt/wt)  6.00      7.00   8.00    9.00 10.00                                  ______________________________________                                        EtCHO    46.20     53.90  61.55   69.04                                                                              76.07                                  n-BtOH   99.84     99.90  99.93   99.95                                                                              99.96                                  n-PrOH   94.32     97.39  98.67   99.24                                                                              99.52                                  ______________________________________                                    

(d) Water washing step

Ethanol was recovered from the extract obtained in the second extractionstep as described in paragraph (c) above by using a column, 3.8 mm ininner diameter and 3 meters in height, having Raschig rings filledtherein, as a water washing column for washing with water andextracting. In this case, the extract obtained in the second extractionstep at the ratio of the extractant of 8 was fed at the rate of 1,200grams per hour from the bottom of the water washing column, and washingwater was fed from the top of the column so as to provide apredetermined ratio of the washing water to the extractant (W/G). Inthis case, the pressure within the column was set to 100 kg per cm² andthe temperature within the column as a whole was maintained at 40° C.The extract was withdrawn from the top of the column, and the lipophilicimpurities were recovered by treating the extract at reduced pressureand vaporizing the extractant only. On the other hand, the raffinate waswithdrawn from the bottom of the water washing column and an aqueousethanol solution was collected by treating the raffinate at reducedpressure and vaporizing the extractant only. The extract and theraffinate were tested for extraction of each ingredient of theimpurities and Table 4 below shows the rates of extraction of eachingredient in the experiments for extraction which were carried out inthe same manner as described hereinabove. The washing rate wasdetermined on the basis of the following formula: ##EQU1##

                  TABLE 4                                                         ______________________________________                                        G/L                                                                           (wt/wt)  0.05     0.06   0.07   0.08  0.09                                    ______________________________________                                        EtOH     64.24    75.85  85.66  92.70 96.83                                   Me--CHO  6.81     8.17   9.54   10.90 12.26                                   i-AmOH   5.63     6.76   7.88   9.01  10.13                                   n-BtOH   12.37    14.84  17.31  19.79 22.26                                   n-PrOH   27.16    32.58  38.00  43.39 48.76                                   Et--Ace  1.51     1.82   2.12   2.42  2.73                                    MeOH     98.63    99.84  99.98  100.00                                                                              100.00                                  ______________________________________                                    

In removing the lipophilic impurities by extracting the aqueous crudeethanol solution with the carbon dioxide in the liquidized state orcarbon dioxide gases in the supercritical state, the process accordingto the present invention can reduce the ratio of the extractant to aquarter or a half of that to be employed in the conventional processes.Hence, costs of high pressure equipment such as high pressure extractingcolumns, compressors for carbon dioxide gases, and so on, can be reducedto approximately one third of costs required for the conventionalprocesses. Further, power required for the circulation of carbon dioxidegases can be reduced to lower than approximately one third than theconventional processes.

In addition, in instances where a roughly distilled ethanol solution,having the concentration of ethanol of 80% to 90% by weight, is employedas a raw material, the raffinate having the concentration of ethanol offrom 30% to 40% by weight can be obtained in the first extraction step,so that the load to be imposed upon the distilling column can be reducedupon the distillation of the raffinate. When the distilling column ofsuch a type as having a side reboiler is employed and a steam withdrawnfrom the top of the distilling column is concentrated and applied as aheat source for the reboiler or side reboiler in treating the raffinate,energy and utility costs required for the distilling column can bereduced to a great extent.

We claim:
 1. A process for the purification of an aqueous crude ethanolsolution, comprising: (a) a first extraction step wherein said aqueouscrude ethanol solution is extracted with an extractant for removinglipophilic impurities, except for C₃ -C₄ alcohols, contained in theaqueous crude ethanol solution by subjecting said aqueous crude ethanolsolution to extraction with the extractant in a pressurized state, whichcomprises carbon dioxide in a liquidized state or carbon dioxide gas ina super-critical state; (b) a concentration-distillation step wherein araffinate obtained in said first extraction step is fed to a distillingcolumn to thereby obtain a highly concentrated aqueous ethanol solutionfrom a top of the distilling column and withdraw a fraction containingC₃ -C₄ alcohols from the distilling column as a side stream; (c) asecond extraction step wherein the side stream is brought into contactwith the extract in a pressurized state, obtained in said firstextraction step, to thereby extract the C₃ -C₄ alcohols contained in theside stream into the extract; and (d) a water washing step wherein theextract in a pressurized state, obtained in the second extraction step,is brought into countercurrent contact with water in a pressurized stateunder such a condition that a ratio of the weight of the water to theweight of the extract is set to 0.3 or less to thereby recover theethanol contained in the extract into an aqueous phase.
 2. A process forthe purification of an aqueous crude ethanol solution as claimed inclaim 1, wherein the raffinate obtained in said second extraction step(c) is circulated to said first extraction step (a).
 3. A process forthe purification, of an aqueous crude ethanol solution as claimed inclaim 1, wherein an aqueous phase obtained in said water washing step(d) is mixed with said side stream obtained in saidconcentration-distillation step (b).
 4. A process for the purificationof an aqueous crude ethanol solution as claimed in claim 1, wherein saidside stream obtained in said concentration-distillation step (b) ismixed with water.
 5. A process for the purification of an aqueous crudeethanol solution as claimed in claim 2, wherein an aqueous phaseobtained in said water washing step (d) is mixed with said side streamobtained in said concentration-distillation step (b).
 6. A process forthe purification of an aqueous crude ethanol solution as claimed inclaim 2, wherein said side stream obtained in saidconcentration-distillation step (b) is mixed
 7. A process for thepurification of an aqueous crude ethanol solution as claimed in claim 3,wherein said side stream obtained in said concentration-distillationstep (b) is mixed with water.